61 



fill it : the roof, too, or at least that portion adjacent to the mouth of 

 the cavern, may be similarly affected. The rocks in which these 

 caverns occur may be of any geological age. 



2nd Period. By the gradual or quick upheaval of the strata in 

 which these caverns occur, they may become dry. 



During the rising, or at a later period, fragments of rock may 

 have accumulated at the open mouth of the lower cavern, and thus 

 have stopped up its entrance, leaving the roof, sides, and floors 

 bearing evident traces of having been an ocean cave. 



3rd Period. Ages may have elapsed during which other strata 

 may have been depositing in other portions of our globe. But 

 ultimately the earth became inhabited by those ancient, but now 

 extinct mammalia, whose remains abound in its caverns. 



Fig. 3. 



4th Period. Ages may again have intervened when man, in his 

 first rude state of existence, entering this deserted den through the 

 opening its former occupants had used, might have been glad to 

 shelter himself from an inclement climate in this bone-house of a 

 more ancient world. 



During this period traces of man's skill would probably be left 

 within his miserable abode ; pottery, if he possessed the art ; char- 

 coal or charred wood, if he were acquainted with fire ; rude cutting 

 instruments of flint or other hard stone, perhaps spear or arrow- 

 heads. 



